Electrodynamic electro-acoustic transducers are used as acoustic components of electronic apparatuses, such as mobile phones. The electrodynamic electro-acoustic transducer is configured to include a permanent magnet, a voice coil, and a vibration film. The electrodynamic electro-acoustic transducer generates sound waves by vibrating the vibration film, such as an organic film, fixed to the voice coil, by operation of a magnetic circuit of a stator using a magnet.
In addition to the electrodynamic electro-acoustic transducer, an electro-acoustic transducer which uses piezoelectric ceramics for the vibration film is also known. In the electro-acoustic transducer, the piezoelectric ceramics with piezoelectric properties vibrate when an electric signal is applied to thereby generate sound waves.
A high-frequency-range limiting frequency in the electrodynamic electro-acoustic transducer is low, while the use of the electro-acoustic transducer using piezoelectric ceramics is limited to reproduction of high-pitched sound. Therefore, examples of an electro-acoustic transducer formed by combining both the electro-acoustic transducers are disclosed in Patent Documents 1 to 3.
The electro-acoustic transducer disclosed in Patent Document 1 has a structure where a piezoelectric element is bonded in the middle of a diaphragm. Since the piezoelectric element has a mass, inertial force acts to reduce a fundamental-mode frequency of the diaphragm. In addition, since the middle portion of the diaphragm, in which a piezoelectric element is bonded, and its periphery have different rigidities, a frequency of a secondary vibration mode becomes high due to piston movement by the piezoelectric element. For this reason, the electro-acoustic transducer disclosed in Patent Document 1 realizes an increase in the bandwidth of output sound waves.
The electro-acoustic transducer disclosed in Patent Document 2 also has a structure where a piezoelectric element is bonded in the middle of a diaphragm. By using the piezoelectric element for the treble region and the electrodynamic electro-acoustic transducer for the bass region, the electro-acoustic transducer disclosed in Patent Document 2 realizes an increase in the bandwidth of output sound waves.
The electro-acoustic transducer disclosed in Patent Document 3 has a structure where the piezoelectric body is provided in a duct cap of the electrodynamic electro-acoustic transducer. The electro-acoustic transducer disclosed in Patent Document 3 also realizes an increase in the bandwidth of output sound waves by using the piezoelectric body for the treble region and the electrodynamic electro-acoustic transducer for the bass region.
In addition, an example of a composite piezoelectric speaker is disclosed in Patent Document 4. The composite piezoelectric speaker disclosed in Patent Document 4 is a composite piezoelectric speaker with a diaphragm obtained by forming electrodes on upper and lower surfaces of the sheet-like composite piezoelectric body formed of flexible resin and a piezoelectric element, and the electrodes are formed of resin mixed with conductive powder. The characteristics in a high frequency band are improved by forming the electrodes themselves of the same material as for the composite piezoelectric body.